The present invention relates to the field of optical disc manufacturing. More specifically, the present invention relates to a glassmastering photoresist method and system making use of a read-after-write approach for inspecting and insuring master quality.
Disc-shaped recorded media, for example and optical disc or a magneto-optical disc, utilizing the method of optical or magneto-optical signal recording and reproducing, are well known in the art. The various types of such disc-shaped recording media include: (1) recording media of the read-only-memory (ROM) type, e.g., an audio compact disc (CD) or a CD ROM, (2) write-once type recording media on which data can be written only once by the user, and (3) overwrite type recording media, such as a magneto-optical disc, on which data re-recording is possible. Other well known examples of optical recording media are video CDs, CD-I, CD-MO, MD, etc.
In order to mass produce optical discs a master disc is produced through a mastering process. This mastering process involves a basic sequence of processing steps. The sequence starts with providing a circular glass substrate that must be cleaned and dried and then carefully inspected for imperfections, surface smoothness and the like. An adhesive coating layer is then placed on one surface of the glass to exacting tolerances. The adhesive coating layer is followed by a photoresistive coating layer evenly and continuously applied over the adhesive layer over the entire glass substrate surface.
The glass substrate must then be appropriately cured, after which recording with an optical beam takes place. The recording process involves selectively exposing the photoresistive coating to a beam of light from a light source in order to form the appropriate pattern of pits and lands which represent the data recorded on the disk. If the light source is a laser, it is typically of a continuous wave type, with exposure of its beam to the photoresistive coating conventionally controlled by an acousto-optical modulator (AOM).
The AOM acts as an electronic shutter to the laser beam and, as is well-known by those skilled in the art, is controlled by a string of binary 1""s and 0""s generated by an encoder. The encoder converts, for example, stereo audio signals typically recorded digitally on a tape in a video or audio format to the appropriate binary 1""s and 0""s by performing eight-to-fourteen modulation (EFM). As part of this encoding, the encoder generates so-called RS parity bytes and adds merging bits.
After the photoresist has been selectively exposed to the laser, the photo-resistive coating must be developed so that the exposed portions can be removed. Developing is accomplished by placing the substrate in a caustic sodium hydroxide solution. After developing, the glass must be inspected by measuring, among other things, the diffraction orders of the tracks.
The next step, metalization, involves placing a thin coating of silver or nickel over the entire surface of the substrate. This metalization cover follows the pattern of pits and lands of the now developed photo-resistive layer. In the case of silver, the metalization step is accomplished by a well known evaporation process resulting in the formation of a metal coating typically 120 nanometers thick. At this point, the metalized substrate, or the xe2x80x9cglass master,xe2x80x9d is typically xe2x80x9cplayedxe2x80x9d in a specially adapted CD player to insure that the pits have been properly formed.
A nickel plating is then formed via electroplating over the metalized glass substrate such that a metal master is formed when the nickel plating is removed from the substrate. The metal master has pits and lands which are the inverse of those on the metalized glass plate. This metal master is called a xe2x80x9cfatherxe2x80x9d and is created by well-known electroplating procedures. Because the extremely thin silver or nickel metalization forming the glass master adheres to and is removed with the nickel plating of the father, there can only be one father.
From the metal master or father, a so-called xe2x80x9cmotherxe2x80x9d is formed, also out of nickel. The mother is simply the inverse of the metal master or father and is similarly formed by electroplating. Several mothers can be formed from the metal master or father.
Finally, nickel stampers are formed from the mothers. The stampers carry the pattern of the father and are again simply the inverse of the mother. Nickel stampers are formed by electroplating a mother in the same way a mother is formed from the father. From the stampers, compact discs can be manufactured by injecting molten molding resin, e.g., optical quality polycarbonate, at high pressure into a mold comprising the stamper and allowing it to solidify.
The object of the invention is to provide a glassmastering photoresist read-after-write method and system that, while keeping all the advantages of the prior art methods and systems, would at the same time be free of their deficiencies.
According to the present invention, a method for manufacturing an optical disc master is provided in which a stream of digital information is converted to a plurality of pits and lands representing the digital information in the optical disc master. The method starts by providing a glass substrate that is then coated with a layer of photoresist. After that, a layer of alkali material is applied atop the glass substrate with the layer of photoresist thereon. The glass substrate coated with the photoresist layer and alkali layer on top of it is placed in an environment with a higher than normal humidity. Preferably, the higher than normal humidity is about 90+RH.
Next, the surface of the coated glass is exposed to a writing beam from a light source which is controlled by the digital information to be written to the disc. In the course of this controlling, a focal point of the writing light beam moves relative to the surface in a pattern so as to allow for formation of the pits and lands. To optimize pit and land geometry, the writing laser beam may be adjusted.
A chemical interaction is initiated at exposed portions of the coated substrate between an exposed photoresist and the alkali material in the humid environment. According to the present invention, the exposed portions are developed and ablated thereby creating the pits, whereas unexposed portions of the coated substrate are kept unaltered thereby creating the lands.
To enhance the bond between the glass substrate and the photoresist layer, a barrier layer material is applied between the glass substrate and the layer of photoresist. The barrier layer on top of the glass substrate is preferably deposited by spin coating.
Preferably, the barrier layer material is titanium acetyl acetonate. Preferably, the alkali material is NaOH. Alternatively, the alkali material can be KOH.
To enhance adhesion of the alkali material to the photoresist layer, the alkali material is combined with an adhesion promoter. Preferably, the adhesion promoter is an optically clear gel, and the thickness of the layer of alkali material combined with the adhesion promoter is selected to be about 120 nm. Alternatively, the thickness of a layer of the alkali material combined with the adhesion promoter may be more than 120.
The moving of the focal point of the writing light beam relative to the coated substrate is accomplished by rotating the coated substrate and simultaneously translating the writing light beam radially. A linear velocity of the moving of the focal point relative to the coated substrate is constant.
Preferably, the writing light beam is a blue or UV laser beam, and a radiation wavelength range thereof is selected between about 351 nm and about 458 nm. Alternatively, the writing light beam may be a blue beam of a LED with a radiation wavelength range between about 442 nm and about 458 nm.
Also, the photoresist preferably comprises a combination of Novolac and diazide, the diazide transforming into a ketene component with the releasing of a gaseous nitrogen component in response to the exposure to the writing light beam. The chemical interaction caused by the writing light beam further comprises formation of an organic acid from the ketene component and water, with subsequent formation of an organic salt from the organic acid and the alkali material.
A reading light source operating from the bottom of the glass structure can also be provided after the exposure. The beam of the reading light source is provided at an appropriate energy and intensity so that it cannot expose the photoresist layer. A couple of photodetectors means are provided to receive the reading beam as transmitted through the glass master.
The first photodetector is in alignment with the reading beam, whereas the second photodetector is shifted relative to the first photodetector so as to detect diffraction of the reading beam. A focal point of the reading beam is moved relative to the glass substrate in a second pattern as the focal point of the writing beam is controlled. Thus, the reading beam determines whether a particular portion of the photoresist has been developed.
The reading light source is preferably a red HeNe laser radiating at a wavelength of about 635 nm. Alternatively, the reading light source can be a red LED radiating at a wavelength of about 635 nm. The diffraction of the reading beam detected by the second photodetector is preferably a first order diffraction. For that reason, the second photodetector may be positioned about 23xc2x0 of the optical axis of the first photodetector.
To ablate the exposed portions of the substrate coating, wet vacuum is applied to the coated substrate and moved over the entire coated substrate.
A system for recording information in a read-after-write process of making optical discs comprises according to the present invention: a glassmaster, a recording chamber accommodating the glassmaster, first light emanating means for writing information on the glassmaster, and second light emanating means for reading the information written by the first writing means.
The glassmaster comprises a glass substrate coated with a first layer of a photoresist and with a second layer of an alkali material, the second layer being applied atop the first layer. There is humidity created in the recording chamber, the level of the humidity being sufficient enough to condense water atop the layer of alkali material. The information being written on the glassmaster is a representation of digital information to be recorded on the optical discs. The digital information controls the first light emanating means so that it exposes the photoresist in accordance therewith. There is also a photodiode couple cooperating with the second light emanating means.